The approval of belatacept, a 2rd generation CD28 pathway costimulation blocker, represents a new class of immunosuppressive agents. The design, translation and clinical evaluation of belatacept was the culmination of efforts by industry and academia to rationally engineer a new therapeutic. The result is an agent that better preserves renal function, reduces donor-specific antibody formation and mitigates cardiovascular side effects. Despite these significant advances, important challenges remain as belatacept treatment is associated with higher rates and grades of rejection. In addition, current immunosuppressive strategies, including belatacept-based regimens, lack the ability to specifically target donor-reactive responses resulting in a generalized state of immune compromise and infectious susceptibility. Our early studies in mice and non-human primates (NHP) identified costimulation blockade resistant rejection (CoBRR) as an important area for investigation and emphasized the potential benefits of developing a successful tolerance strategy. The recent clinical results with belatacept highlight the pressing need to understand the cellular and molecular pathways that contribute to CoBRR and to develop targeted strategies to overcome them, including clinically applicable tolerance regimens. In the first aim of our project we will evaluate novel 3rd generation costimulation blockade therapeutics in collaboration with our partners in industry including domain antibodies designed to safely and specifically block both the CD28 and CD40 pathways. We anticipate that there will still be a CoBRR response and have new data from preliminary murine and NHP studies suggesting a critical role for the IL-17 pathway. We have also identified a successful strategy utilizing an antibody targeting both IL-12 and IL-23 to control this break-through rejection response. In aim 2 we will further test this novel IL-12/23 therapy in combination with optimized 3rd generation costimulation blockade. We will synthesize observations from the first two aims to define a successful therapeutic regimen that we will then couple to our novel pulsed donor antigen tolerance strategy. Results from these studies will help develop a clinically translatable tolerance strategy, which avoids the morbidity and mortality of chronic continuous immunosuppression. The new knowledge obtained in these studies will be significant and wilt contribute to improved outcomes, enhanced quality of life, and reduced morbidity by avoiding rejection, simplifying regimens, and avoiding infection as a result of long-term immunosuppression.